Halving the Barrier to Gas-Phase Oxidation of Bromide by Ozone

Recent atmospheric measurements have indicated that bromine and iodine may be responsible for up to 72% of halogen-induced ozone loss near the tropopause. Whilst the neutral bromine and iodine radicals have well-described reaction pathways with ozone, there is ongoing uncertainty regarding the multiphase chemistry of the corresponding bromide and iodide anions and thus the potential role of these ionic species in ozone depletion. Here, we demonstrate the unequivocal ozone-dependence of the archetype Br¯ + O3 reaction, which proceeds at ambient temperatures with an experimental rate constant of 8.9 (±4.4) × 10-15 cm3 molecule-1 s-1 (0.001% collision efficiency). Using a state-of-the-art computational approach, the reaction mechanism is revised to proceed via a singlet transition state with a calculated barrier of +22.1 kJ mol-1, substantially lower than prior estimates and not on the triplet surface as previously claimed. Statistical rate modelling using this new barrier height predicts a rate constant of 5.7 × 10-15 cm3 molecule-1 s-1 from canonical transition state theory, which is in excellent agreement with the experiment. This reconciliation of the kinetics for the intrinsic gas-phase reaction will enable systematic evaluation of temperature, pressure and solvation effects on this ion-molecule chemistry and thus inform the impact of both bromide and iodide anion chemistry on atmospheric ozone.